Existing agents such as estrogen, bisphosphonates, fluoride, or calcitonin can prevent bone loss and induce a 3–5% increase of bone mass by refilling the remodeling space, but net bone formation is not significantly stimulated. The retention of bone by inhibition of bone turnover may not be sufficient protection against fracture risk for patients who already have significant bone loss. Anabolic agents that increase bone strength by stimulating bone formation preferentially may provide better protection against fracture in patients with established osteoporosis.
Parathyroid hormone (PTH) is a secreted, 84 amino acid product of the mammalian parathyroid gland that controls serum calcium levels through its action on various tissues, including bone. The N-terminal 34 amino acids of bovine and human PTH (PTH(1-34)) is deemed biologically equivalent to the full length hormone. Other amino terminal fragments of PTH (including 1–31 and 1–38 for example), or PTHrP (PTH-related peptide/protein) or analogues of either or both, that activate the PTH/PTHrP receptor (PTH1 receptor) have shown similar biologic effects on bone mass, although the magnitude of such effects may vary.
Studies in humans with various forms of PTH have demonstrated an anabolic effect on bone, and have prompted significant interest in its use for the treatment of osteoporosis and related bone disorders. The significant anabolic effects of PTH on bone, including stimulation of bone formation which results in a net gain in bone mass and/or strength, have been demonstrated in many animal models and in humans.
It is commonly believed that PTH administration in humans and in relevant animal models has a negative effect on cortical bone. In fact, naturally occurring increases in endogenous PTH, which occur in the disorder hyperparathyroidism, result in thinning of cortical bone accompanied by an increase in connectivity and mass of trabecular bone. Past studies suggest that when Haversian cortical bone (found in humans and higher mammals) remodels under the influence of PTH, there will be a re-distribution of bone such that cortical bone mass and strength decrease, while trabecular bone increases in mass and strength. For example, in published clinical studies of administering PTH, cortical bone mass decreased after treatment with exogenous PTH and these findings have raised concern that the treatment of PTH will lead to reduced cortical bone mass and strength. One concern raised by such studies is that there would be a loss of total skeletal bone mass due to the loss of cortical bone. This is of high clinical relevance as, in osteoporosis, the greater loss of trabecular bone compared to loss of cortical bone, means that mechanical loading is predominantly borne by the remaining cortical bone. Continued loss of cortical bone would increase the fracture risk. Therefore, it is important that a therapeutic agent for osteoporosis maintain or increase a subjects residual cortical bone.
The effects of PTH on cortical bone have been investigated in nonhuman animals with Haversian remodeling, such as dogs, ferrets, sheep and monkeys, but sample sizes are typically too small for reliable statistical analysis. The impact of the changes induced by PTH treatment on mechanical properties of cortical bone in such animals remains unknown. Published studies of rodents have shown increased cortical bone mass during administration of PTH but a loss of this benefit after withdrawal of PTH. However, rodent cortical bone has a distinctly different structure from Haversian cortical bone, and remodels by surface appositional formation and resorption, rather than by intracortical remodeling of osteons. Furthermore, technological limitations in biomechanical testing on the relatively short bones of rodents give rise to artifacts of measurement when an agent, such as a PTH, alters bone geometry to thicken the bone. Such artifacts make extrapolation of rat cortical bone responses to those of humans or other animals with osteonal remodeling unreliable. Therefore, the existing data for animals, like humans, undergoing Haversian remodeling indicates that PTH may have an adverse impact on cortical bone, causing net loss of bone mass through depletion of cortical bone.
As a consequence, it has been a popular belief regarding the action of PTH that patients require concurrent or subsequent treatment with an antiresorptive to minimize loss of bone induced by PTH. In fact, this model has been the basis for several clinical studies in women. For example, three clinical studies have used PTH in post-menopausal women on concurrent therapy with calcitonin or estrogen, or in premenopausal women taking GnRH agonist, Synarel, for endometriosis. The opposing effects of estrogen and PTH on cortical bone turnover make it particularly difficult to observe effects of just PTH during combination therapy with these two agents.
There remains a need for a method for employing a PTH to increase strength and stiffness of bone in humans and other animals exhibiting Haversian remodeling, and for reducing the incidence of fracture of bones in these animals. Furthermore, there remains a need for a method for increasing the quality and amount of cortical bone.